For a meditator following the vehicle of serenity the attainment of jhana fulfills two functions: first, it produces a basis of mental purity and inner collectedness needed for undertaking the work of insight contemplation; and second, it serves as an object to be examined with insight in order to discern the three characteristics of impermanence, suffering and non-self. Jhana accomplishes the first function by providing a powerful instrument for overcoming the five hindrances. As we have seen, for wisdom to arise the mind must first be concentrated well, and to be concentrated well it must be freed from the hindrances, a task accomplished pre-eminently by the attainment of jhana. Though access concentration will keep the hindrances at bay, jhana will ensure that they are removed to a much safer distance.
In their capacity for producing concentration the jhanas are called the basis (pada) for insight, and that particular jhana a meditator enters and emerges from before commencing his practice of insight is designated his padakajjhana, the basic or foundational jhana. Insight cannot be practiced while absorbed in jhana, since insight meditation requires investigation and observation, which are impossible when the mind is immersed in one-pointed absorption. But after emerging from the jhana the mind is cleared of the hindrances, and the stillness and clarity that then result conduce to precise, penetrating insight.
Some commentators have suggested that, in practice, this fivefold perspective can be gained simply by focusing on the arising and passing away of these aggregates in the present moment; if one's focus is relentless enough, it will lead naturally to a knowledge of drawbacks, allure, and escape, sufficient for total release. The texts, however, don't support this reading, and practical experience would seem to back them up.
vipassana needs to be teamed with samatha — the ability to settle the mind comfortably in the present — so as to master the attainment of strong states of absorption, or jhana. Based on this mastery, samatha and vipassana are then applied to a skillful program of questioning, called appropriate attention, directed at all experience: exploring events not in terms of me/not me, or being/not being, but in terms of the four noble truths. The meditator pursues this program until it leads to a fivefold understanding of all events: in terms of their arising, their passing away, their drawbacks, their allure, and the escape from them. Only then can the mind taste release.
http://www.accesstoinsight.org/lib/auth ... etool.html